3GPP Long Term Evolution, LTE, is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. In a typical cellular radio system, wireless devices also known as mobile stations and/or user equipment units, UEs, communicate via a radio access network, RAN, to one or more core networks. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a wireless device is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNB or eNodeB, in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a wireless device and receiving signals transmitted by a wireless device.
An uplink transmission of a wireless device can, in general, be received at several base stations, although typically a single base station controls the uplink transmission of the wireless device.
State-of-the art wireless access systems typically employ an Automatic Repeat Request mechanism, e.g. a Hybrid Automatic Repeat Request (HARQ) mechanism to increase the spectral efficiency of the system. Such a mechanism uses feedback messages sent from the data receiver to a data sender in order to trigger retransmissions if the previous transmission failed.
In order to minimize the transmission cost and delays of the ARQ respectively HARQ feedback, state-of-the-art ARQ/HARQ mechanisms employ a fixed timing relation between the transmission of the signal (from the sender to the receiver) and the transmission of the ARQ/HARQ feedback (from the receiver to the sender). An example of this is the HARQ protocol in LTE as defined in the MAC protocol specification. HARQ timing for uplink is described in 3GPP TS 36.213 version 12.3.0 section 10.1. HARQ timing for downlink is not strict in LTE, but the maximum processing time under which both HARQ and full Transmission Time Interval, TTI, utilization can be achieved can be derived from the number of parallel HARQ processes which can be found in 3GPP TS 36.213, section 7, version 12.3.0. The HARQ retransmission protocol in Frequency Division Duplex, FDD, mode is designed for a round-trip time of 8 ms. For TDD the round trip time, RTT, depends on TDD configuration. For some TDD configurations the RTT can be different depending on HARQ process
In LTE, it is also specified that the HARQ feedback needs to be sent after a certain number of Transmission Time Intervals, TTIs, or equivalent sub-frames. For LTE FDD the feedback needs to be sent 4TTIs after the transmission. This implies tight timing constraints for the uplink HARQ protocol operation. Achieving full throughput for one UE requires a processing latency of at most 3 ms on the network side, from the reception of an uplink signal until the transmission of a potential downlink response signal.